In recent years, with the progress of various information devices, the storage capacity of magnetic recording media has increased more and more. Particularly, the recording capacity and recording density of magnetic disks, which play a central role as external memories in computers, have been increasing year by year. Under such circumstances, there is a need for development of a magnetic disk which enables higher-density recording. For example, development of laptop and palmtop personal computers has required a small-sized recording apparatus with high impact resistance, and therefore, demand has arisen for a small-sized magnetic recording medium which enables higher-density recording and has high mechanical strength. Recently, navigation systems and portable music playback devices have also employed a recording apparatus incorporating an ultra small magnetic recording medium.
Conventionally, such a magnetic recording medium (i.e., magnetic disk) has employed an aluminum alloy substrate having an NiP-plated surface, or a glass substrate, which satisfies strict requirements, including higher impact resistance, rigidity, hardness, and chemical durability. Such a glass substrate is advantageous in that it enables easy formation of a flat surface suitable for reduction of the flying height of a magnetic head flying above a magnetic recording surface, the flying height reduction being important for attaining high-density magnetic recording.
In recent years, the flying height of a magnetic head flying above a magnetic recording medium has been reduced more and more for attaining high-density recording, and in accordance with this tendency, a substrate for forming a magnetic recording medium (hereinafter may be referred to as a “magnetic recording medium substrate” or a “magnetic recording medium substrate product”) has been required to have a flat surface with small surface roughness (e.g., a mirror surface) and to have minimized surface defects such as microscratches and micropits.
A magnetic recording medium substrate (e.g., a hard disk (HD) substrate) is produced through the following steps: a step of roughly polishing (lapping) the surface of a substrate body which has been formed to have desired dimensions; a step of chamfering the inner and outer end surfaces of the substrate body through grinding; and a step of subjecting the substrate body surface to final polishing. Subsequently, layers are formed atop the substrate body surface through, for example, the following steps: a texturing step in which the substrate body surface is moderately roughened; a step of forming an underlayer on the surface of the substrate; a step of forming a magnetic layer on the surface of the underlayer; a step of forming a protective layer on the magnetic layer; and a step of forming a lubrication layer on the protective layer. Thereafter, irregular protrusions formed on the layer surface are removed through a burnishing step, to thereby yield a magnetic recording medium as a final product.
The thus-produced substrate may cause a problem in that exfoliation of the underlayer or the magnetic layer occurs at a chamfered portion of the substrate body, resulting in production of defective products. In order to solve this problem, there has been proposed means for enhancing the durability of the layers formed atop the substrate body, in which the average surface roughness (Ra) of the chamfered portion is regulated to less than 0.20 μm (see, for example, Patent Document 1).
In the case where the flying height of a magnetoresistive head is reduced for enhancing recording density, when data recorded on a magnetic recording medium are reproduced by the head, a reproduction error may occur, or the data may fail to be reproduced. Such a phenomenon is caused by the following mechanism: projections formed of particles deposited on the surface of the magnetic recording medium induce thermal asperity and generate heat in the magnetoresistive head, and such heat generation varies the resistance of the head, which adversely affects electromagnetic conversion characteristics. Particles which adversely affect flying of the head (e.g., angular particles or particles which rise relatively sharply with respect to the medium (disk) surface) were found to be generated through unintentional rubbing of the end surfaces of a magnetic recording medium substrate with a storage cassette when the substrate is stored in the cassette and conveyed during production of the magnetic recording medium. Conceivably, when the substrate, whose end surfaces (including a chamfered portion and a substrate side wall) do not assume mirror surfaces, is placed into or removed from the storage cassette formed of polycarbonate or a similar material, the substrate end surfaces come into contact with the inner wall of the storage cassette, and the aforementioned particles are generated from the end surfaces and are deposited on the surface of the magnetic recording medium. In order to solve such a problem, there has been proposed a technique for regulating the average surface roughness (Ra) of the surface of at least one of the chamfered portion and the side wall of the substrate to less than 1 μm; i.e., a technique for forming a mirror surface (see, for example, Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 09-102120    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 10-154321